Constrain to any 2 axes during transform?
While Blender doesn't have widgets to do this, its fairly straightforward.


Using the Manipulator: Hold Shift and click on the Grab/Scale handles,
X locks YZ,
Y locks XZ,
Z locks XY
Using Transform keys: Grab with G or Scale with S,
ShiftX locks YZ,
ShiftY locks XZ,
ShiftZ locks XY


As @sambler notes, you can do G Shift X,X move along the objects Local YX axis, works for any axis of course.